To support the wheels so as to be rotatable with respect to the suspension, there are used various axle units in which outer and inner rings are rotatably combined together through rolling elements. An axle unit for a driving wheel—which is used not only to support the drive wheels on the independent suspension, but also to drive and rotate the drive wheels—in combination with a constant velocity joint, must be able to transmit the rotary motion of the drive wheels smoothly (that is, in such a manner that the constant velocity performance can be secured) regardless of not only the mutual shift between the differential gears and drive wheels but also the steering angles applied to the wheels. As an axle unit for a driving wheel referred to as a so called fourth-generation hub unit, which, in combination with a constant velocity joint, can be structured so as to be relatively compact and light in weight, there is conventionally known an axle unit disclosed in JP-A-7-317754, or an axle unit disclosed in the specification of U.S. Pat. No. 5,674,011.
Now, FIG. 10 shows a first example of the conventional structure disclosed in the above-cited JP-A-7-317754. According to this structure, the axle unit is assembled to a vehicle and is supported on the suspension. In this state, an outer ring 1, which is disposed so as not to rotate, includes in the outer peripheral surface thereof an outwardly-facing flange-shaped mounting portion 2 to be supported on the suspension and, in the inner peripheral surface thereof a double row of outer ring raceways 3, 3. On the inside diameter side of the outer ring 1, there is disposed a hub 6 which is composed of a combination of a first element 4 and a second element 5. Of the two elements, the first element 4 is formed in a cylindrical shape. Specifically, in the cylindrical-shaped first element 4, on the direction of an outer-end (in the present specification, the outer end with respect to the width direction of the axle unit when it is assembled to a vehicle; in FIG. 10, the left end) portion of the outer peripheral surface thereof, there is formed a mounting flange 7 which is used to support the wheel; and, in the direction of an inner-end (in the present specification, the central end with respect to the width direction of the axle unit when it is assembled to a vehicle; in FIG. 10, the right end) portion of the outer peripheral surface thereof, there is formed an inner raceway 8. On the other hand, in the case of the second element 5, one end portion (in FIG. 10, the left end) thereof is formed as a cylindrical portion 9 to which the outer surface of the first element 4 can be fitted and fixed; the other end portion (in FIG. 10, the right end) thereof is formed as a housing portion 11 which defines the outer ring of a constant velocity joint 10 of a Zwepper type or a bar field type; and, in the outer peripheral surface of the middle portion thereof, there is formed an inner raceway 8. A plurality of rolling elements 12, 12 are interposed between the above-mentioned outer raceways 3, 3 and inner raceways 8, 8, whereby the hub 6 is rotatably supported inside the outer ring 1.
Also, in the mutually matched positions of the inner peripheral surface of the first element 4 and the outer peripheral surface of the second element 5, there are formed an outside securing groove 13 and an inside securing groove 14 respectively. A retaining ring 15 is bridgingly interposed between the two securing grooves 13, 14. In this manner, the first element 4 is prevented from slipping out of the second element 5. Further, a weld 17 is produced between the outer peripheral edge portion of the outer end face (in FIG. 10, the left end face) and the inner peripheral edge portion of a stepped portion 16 formed in the inner peripheral surface of the first element 4, thereby connecting and fixing the first and second elements 4, 5 to each other.
Further, between the two end opening portions of the outer ring 1 and the outer peripheral surface of the middle portion of the hub 6, there are interposed not only substantially cylindrical-shaped covers 18, 18 made of metal plates such as stainless steel plates, but also circular-ring-shaped seal rings 19, 19 made of elastic material such as rubber or elastomer. Also, on the inside of the middle portion of the second element 5, there is formed a partition plate portion 20 which is used to close the inside of the second element 5. These covers 18, 18, seal rings 19, 19 and partition plate portion 20 not only shut off the portion where the plurality of rolling elements 12, 12 are disposed or the constant velocity joint 10 portion from the outside to thereby prevent grease from leaking to the outside, but also prevent foreign substances such as rainwater and dust from getting into the inside.
The constant velocity joint 10 is composed of the housing 11, an inner ring 21, a retainer 22, and a plurality of balls 23. Of these parts, the inner ring 21 is fixed to the leading end portion of a drive shaft (not shown) which can be driven or rotated through a transmission by an engine. In the outer peripheral surface of the inner ring 21, there are formed a plurality of inside engaging grooves 24 respectively extending at right angles to the circumferential direction of the inner ring 21. Each of the inside engaging grooves 24 has a section shape which provides an arc shape when it is cut by a virtual plane intersecting at right angles to the center axis of the inner ring 21. The inside engaging grooves 24 are arranged at regular intervals in the circumferential direction of the inner ring 21. Also, in the position of the inner peripheral surface of the housing portion 11 that is opposed to the inside engaging grooves 24, there are formed a plurality of outside engaging grooves 25 each similarly having an arc-shaped section shape in such a manner that they extend at right angles to the circumferential direction of the inner ring 21. And, the retainer 22 has an arc-shaped section and is formed in a circular-ring-like shape as a whole. The retainer 22 is held by and between the outer peripheral surface of the inner ring 21 and the inner peripheral surface of the housing portion 11. In the circumferential-direction, six positions of the retainer 22 are matched to the inside and outside engaging portions 24, 25, and in these positions there are formed six pockets 26 respectively. Inside these six pockets 26, there are held a corresponding number of balls 23, a total of six balls 23. These balls 23, while they are retained in their respective pockets 26, are allowed to roll along the inside and outside engaging portions 24, 25.
When assembling the above-structured drive-wheel axle unit to a vehicle, the outer ring 1 is supported on the suspension through the mounting portion 2, and the drive wheels are fixed to the first element 4 through the mounting flange 7. Also, the leading end portion of the drive shaft—(not shown), which can be driven or rotated by the engine through the transmission and differential gear—is spline engaged with the inside of the inner ring 21 forming the constant velocity joint 10. While the vehicle is running, the rotary motion of the inner ring 21 is transmitted through the plurality of balls 23 to the second element 5 and to the hub 6 to thereby drive or rotate the drive wheels.
Now, FIG. 11 shows a second example of a conventional structure as disclosed in the specification of the above-cited U.S. Pat. No. 5,674,011. In the second conventional example, the inner peripheral surface of an outer ring 1 is fitted with and fixed to the inner surface of a knuckle 27 forming the suspension. The outer ring 1 does not rotate when the present axle unit is in use, and in it there are formed a double row of outer raceways 3, 3. On the direction of an outer-end (in FIG. 11, the direction of left-end) portion of the outer peripheral surface of a hub 6a, there is disposed a mounting flange 7 which is used to support the wheels of a vehicle. In the direction of an inner-end (in FIG. 11, the direction of right-end) portion of the outer peripheral surface of the hub 6a, there are formed a double row of inner raceways 8, 8 through a pair of inner rings 28, 28. These two inner rings 28, 28 are supported on and fixed to the main body portion of the hub 6a by a caulk portion 29 formed by plastically deforming the other end portion of the hub 6a in such a manner that it is bent outwardly in the diameter direction of the hub 6a. Between the outer raceways 3, 3 and inner raceways 8, 8, there are interposed a plurality of rolling elements 12, 12; and, the hub 6a is rotatably supported inside the outer ring 1.
In the central portion of the hub 6a, there is formed a spline hole 30. The thus-structured hub 6a is combined with a drive shaft member 31 to thereby provide an axle unit for a driving wheel. On one end portion of the drive shaft member 31, there is disposed a spline shaft 32 which can be engaged with the spline hole 30. And the other end portion of the drive shaft member 31 is formed as a housing portion 11 which provides an outer ring of a constant velocity joint. The thus-structured drive shaft member 31 and hub 6a are combined together in such a manner that the spline shaft 32 is inserted into the spline hole 30, and a connecting member 33 made of elastic material is unevenly engaged with the two members 31, 6a to thereby prevent the two members 31, 6a from being separated from each other.
Of the respective conventional structures that have been described hereinabove, in the first conventional structure example shown in FIG. 10, the rotary power between the first and second elements 4, 5 forming the hub 6 must be transmitted through the weld 17 portion. That is, between the first element 4 supporting the wheels and the second element 5 connected to the drive shaft, it is necessary to transmit large torque for driving. But because the first and second elements 4, 5 are fitted with each other through respective cylindrical surfaces, large torque cannot be transmitted through the fitting surfaces thereof. Therefore, the large torque must be transmitted through the weld 17 portion. In order to increase the strength of the weld 17 portion to a sufficient level, the entire periphery of the weld 17 must be welded in a cladding manner. However, where the entire periphery of the weld 17 is claddingly welded, heat at the welding time can distort the shape of the inner raceway 8 portions formed in the outer peripheral surface of the first element 4, or can lower the hardness of the inner raceway 8 portions, which makes it impossible to secure sufficient durability of a rolling bearing unit including the inner raceways 8.
Also, in the second conventional structure example shown in FIG. 11, since prevention of the mutual separation between the hub 6a and drive shaft member 31 is attained by the connecting member 33 made of elastic material, the function of the separation prevention thereof seems unreliable. That is, when a vehicle turns suddenly, due to a large thrust load and a moment load applied to the hub 6a from the wheels, there is applied a large force in a direction to pull out the hub 6a from the drive shaft member 31. With such a large pull-out force, it is difficult to positively prevent the mutual separation between the hub 6a and drive shaft member 31. Therefore, this structure is believed not to be able to secure sufficient reliability.